1. Field of the Invention
Apparatuses and methods consistent with the present invention relate to an optical system, and more particularly, to a projection optical system which can be used to generate and project an image.
2. Description of the Related Art
In a projection optical system, a light generated by a light source is condensed, by a condensing lens providing homogeneous illumination, on a film and then is projected onto a screen via a lens. In order to form an image, a recent projection optical system (disclosed in Japanese Patent No. 2000-321529, and Japanese Patent No. 2005-092206) does not use the film but uses micromirror devices, such as a digital micromirror device (DMD), which are composed of a set of rotary mirrors. Such a projection optical system includes an illuminating optics such as the set of rotary mirrors for achieving the homogeneous illumination, and a projecting optics such as the lens for projecting the image onto the screen.
However, the projection optical system is disadvantageous in that, due to a relatively long travel distance of light within the projection optical system, a considerable loss of light occurs, and this requires an increase in optical power which may excessively heat the projection optical system. In order to prevent such excessive heat, the overall size of the projection optical system needs to be increased.
According to a projection system disclosed in U.S. Pat. No. 6,439,726, an illuminating optics and a projecting optics of the projection system are divided into first, second and third optical parts. The first and second optical parts have a common optical axis and together compose a projection lens. The second and third optical parts together compose the illuminating optics. An auxiliary illumination light, such as a backlight, is emitted from the third optical part, is illuminated on the second optical part including a micromirror device, forms an image pattern, is emitted from the projection system and then is projected onto the screen after having passed through the first optical part. At this time, by installing the third optical part at an angle to the first and second parts, an optical trajectory of the light, that is, a distance between a light source and the micromirror device is shortened so that loss of light is reduced and the required optical power is reduced. Accordingly, the size of the projection system can be reduced.
However, such a projection system has the following two disadvantages. First, in general, a compact projection system uses a light source, such as a gas-discharge lamp or a light emitting diode, which has great divergence. At this time, the light efficiency of a device, the overall size of the device, and image quality are important features. The higher requirements to the light efficiency along with application of high-aperture light sources necessitate great numerical aperture of the illuminating light beam falling on the micromirror plate and the great numerical aperture of the projection lens. Thus, an aperture diaphragm of the second optical part, wherein the aperture diaphragm is located between the first optical part and the second optical part, represents a pupil plane with respect to both the illumination light and a projection beam. A beam splitting in the pupil plane (with the numerical aperture that is equal to, or that exceeds, sinus of an inclination angle of the micromirror plate) or in its vicinity, minimizes and uniformizes beam vignetting. An acceptable distance depends on the numerical aperture of the lens, that is, the smaller the numerical aperture is, the further displacement of the beam splitting node from the pupil is possible. In general, such a distance does not exceed a diameter of the pupil. For efficient separation of an illumination part and a projecting part, an aberration in the pupil is also important. In other words, coincidence of the position and the size of the pupil for all points of the field is important. Also, pupil quality is important not only for the projecting part, but also for the illuminating part. In the case where the illuminating part has great pupil aberrations, the projection system demonstrates heterogeneity of vignetting across an image area (field), and increase in an ambient light component, such that optical devices overheat and image quality deteriorates. Thus, in order to achieve a uniform brightness and generate a high-quality image in the compact projection system, correction of the pupil of the second optical part is necessary for both the projection beam and illumination beam. By doing so, an adjustable numerical aperture of the second optical part is increased. In this case, an applied aperture is asymmetric with respect to the second optical part when the illumination beam reaches the second optical part at a great angle. On the other hand, the projection beam generally propagates symmetrically or almost symmetrically along an optical axis.
Second, in the compact projection system an image is projected a large diagonal distance, and there is a short distance between a projector and screen. However, the compact projection system has to project an image beyond an optical axis of the projection system for a spectators' convenience. This prevents the casting of spectators' shadows on the screen. For this purpose, the screen has to be placed above the optical axis of the projection system when the projector is located on a table, and the screen has to be placed below the optical axis of the projection system when the projector is located on a ceiling. In order to generate an image on a screen placed beyond the optical axis of the projection system, an asymmetric projection lens is used in a related art image projection apparatus. Here, the set of rotary mirrors is shifted in a vertical direction to an optical axis of the asymmetric projection lens and disposed. Such a disposal requires a significant increase in an adjustable vision field. An adjustable area of an object, that is, an area located in a plane of the set of rotary mirrors can exceed an effective vision field, which is normally equal to the size of the set of rotary mirrors, by two or three times. This seriously complicates lens design calculations, in terms of correction of distortions and aberrations. Thus, costs of calculations and manufacturing of the projection system increase, and the overall size of the projection system also increases.